Does Blender have a way to reduce texture size?
Create a separate project, import the textures into the VSE as an image sequence, lower the dimesion values to 64 px by 64 px, and render them all out as .pngs or your preferred image filetype.

Though as NoviceInDisguise, Timaroberts and Galih Bagaskara have already said, this is probably best handled by GIMP or some other dedicated graphic editor. Using the method I described, every texture will lose its individual name once rendered, which may or may not be important to you.